Determining the geographical position of a mobile station within a wireless cellular network or other Public Land Mobile Network (PLMN) has recently become important for a wide range of applications. For example, positioning services may be desired by transport and taxi companies to determine the location of their vehicles and to improve the efficiency of dispatch procedures. In addition, for emergency calls, e.g., 911 calls, knowing the exact location of a mobile terminal may be vital in ensuring a positive outcome in emergency situations.
Furthermore, positioning services can be used to determine the location of a stolen car, to identify home zone calls which may be charged at a lower rate, to detect hot spots in a micro cell, or to provide premium subscriber services, e.g., the Where Am I service. The Where Am I service facilitates the determination of, for example, the location of the nearest gas station, restaurant, or hospital to a mobile station.
One technique for determining the geographic position of a mobile station is to use the satellite-based Global Positioning System (GPS). GPS is a satellite navigation system that provides specially coded satellite signals that can be processed in a GPS receiver to yield the position, velocity and time of a receiving unit. Four or more GPS satellite signals are needed to compute the three-dimensional locational coordinates and the time offset of a receiver clock relative to a fixed coordinate system.
The GPS system comprises twenty-four satellites (not counting spares) that orbit the Earth in approximately twelve hours. The orbital altitude of the GPS satellites (20,200 km) is such that the satellites repeat the same ground track and configuration over any point approximately once every twenty-four hours. There are six orbital planes each nominally with at least four satellites in each, that are equally spaced (i.e., 60.degree. apart) and inclined at about 55.degree. relative to the equatorial plane to of the Earth. This constellation arrangement ensures that between four and twelve satellites are visible to users from any point on Earth.
The satellites of the GPS system offer two levels of precision in determining the position, velocity and time coordinates at a GPS receiver. The bulk of the civilian users of the GPS system use the Standard Positioning Service (SPS) which has a 2-.sigma. accuracy of 100 meters horizontally, .+-.156 meters vertically and .+-.340 ns time. The Precise Positioning Service (PPS) is available only to authorized users having cryptographic equipment and keys and specially equipped receivers.
Each of the GPS satellites transmits two L-band carrier signals. The L1 frequency (centered at 1575.42 MHz) carries the navigation message as well as the SPS and PPS code signals. The L2 frequency (centered at 1227.60 MHz) also carries the PPS code and is used to measure the ionospheric delay by receivers compatible with the PPS system.
The L1 and L2 carrier signals are modulated by three binary codes: a 1.023 MHz Coarse Acquisition (C/A) code, a 10.23 MHz Precise Code (P-Code) and a 50 Hz Navigational System Data Code (NAV Code). The C/A code is a pseudorandom number (PRN) code that uniquely characterizes a GPS satellite. All of the GPS satellites transmit their binary codes over the same L1 and L2 carriers. The multiple simultaneously-received signals are recovered by a Code Division Multiple Access (CDMA) correlator. The correlator in a civilian GPS receiver first recovers the C/A Code as modulated by the NAV Code. A Phase Locked Loop (PLL) circuit then separates out the C/A Code from the NAV Code. It should be emphasized that a GPS receiver first needs to determine its approximate location in order to determine which of the GPS satellites are actually visible. Conversely, a GPS receiver that knows its approximate position can acquire more quickly the signals transmitted by the appropriate GPS satellites.
The startup of a GPS receiver typically requires the acquisition of a set of navigational parameters from the navigational data signals of four or more GPS satellites. This process of initializing a GPS receiver may often take several minutes.
The duration of the GPS positioning process is directly dependent upon how much information a GPS receiver has. Most GPS receivers are programmed with almanac data, which coarsely describes the expected satellite positions for up to one year ahead. However, if the GPS receiver does not have some knowledge of its own approximate location, then the GPS receiver cannot correlate signals from the visible satellites fast enough, and therefore, cannot calculate its position quickly. Furthermore, it should be noted that a higher signal strength is needed for capturing the C/A Code and the NAV Code at start-up than is needed for continued monitoring of an already-acquired signal. It should also be noted that the process of monitoring the GPS signal is significantly affected by environmental factors. Thus, a GPS signal which may be easily acquired in the open becomes progressively harder to acquire when a receiver is under foliage, in a vehicle, or worst of all, in a building.
Recent governmental mandates, e.g., the response time requirements of the FCC Phase II E-911 service, make it imperative that the position of a mobile handset be determined accurately and in an expedited manner. Thus, in order to implement a GPS receiver effectively within a mobile terminal while also meeting the demands for fast and accurate positioning, it has become necessary to be able to quickly provide mobile terminals with accurate assistance data, e.g., local time and position estimates, satellite ephemeris and clock information (which may vary with the location of the mobile station). The use of such assistance data can permit a GPS receiver that is integrated with or connected to a mobile station to expedite the completion of its start-up procedures. It is therefore desirable to be able to send the necessary assistance GPS information over an existing wireless cellular network to a GPS receiver that is integrated with or connected to a mobile terminal.
It is presently known to provide satellite ephemeris and clock correction information to a remote GPS receiver over a radio link. Likewise, it is common in land surveying to provide Differential GPS (DGPS) corrections over a radio link to remote GPS receivers. However, none of these prior systems address the specific operation requirements of a cellular mobile station and the wireless cellular network with which it interacts.
With a GPS-equipped Mobile Station (GPS-MS), standby time and talk time are limited by battery capacity. The additional battery drain resulting from operation of the integrated GPS receiver can be substantially greater than for the basic cell phone requirements. This can undesirably limit both standby time and talk time.
Providing GPS assistance information to the GPS-MS improves the sensitivity, Time-To-First-Fix (TTFF), and power consumption of the GPS-MS compared to a stand-alone GPS receiver. However, typical GPS-MS usage scenarios pose problems related to obtaining and updating GPS assistance information from the wireless cellular network. For example, the DGPS correction data is very time sensitive and requires frequent updates, which places a burden on the facilities of the wireless cellular network. Also, once new ephemeris and clock correction data is available for a satellite, all GPS-MS for which the satellite is visible require the new assistance as soon as possible in order to maintain a high degree of position accuracy. Timely delivery of these updates can place significant burden on the facilities of the wireless cellular network.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.